1. Field of the Invention
The invention relates to a non-invasive method and device of measuring the pressure of fluid fluctuating in an elastic tube, particularly to a method and device of measuring the actual continuous pressure of fluid inside the elastic tube and the dynamic compliance of the elastic tube.
2. Description of the Related Art
The related techniques of the invention are the measurements of the instantaneous blood pressure in artery and the compliance of the blood vessel. The parameters of the blood pressure and arterial compliance are crucial for diagnosis of human health. The present developments are described as follows:
Methods to Measure the Arterial Blood Pressure
Since Marey (1876) first invented the sphygmograph to measure the blood pressure, many researchers have been trying to develop a non-invasive, convenient and reliable instrument to measure the blood pressure in an artery for medicine and health care. In the clinic, blood pressure is measured almost exclusively using non-invasive intermittent techniques, of which the auscultatory and the computerized oscillometric method are most often used. However, both methods only provide a momentary value for systolic blood pressure (SBP), diastolic blood pressure (DBP) and mean blood pressure (MBP).
Conversely, there are three methods to non-invasively detect the instantaneous arterial blood pressure: arterial tonometry based on the coplanar measurement (Pressman and Newgard 1963, Mackay 1964), Pen{tilde over ( )}n´az's method (Pen{tilde over ( )}n´az 1973, Wesseling 1984, Wesseling and Pen{tilde over ( )}n´az 1986) and the volume-compensation method (Yamakoshi et al 1979, 1980). The latter two are based on the vascular unloading technique (Geddes 1970, Shirer 1962, O'Brien and O'Malley 1991).
Radial artery tonometry such as HDI CvProfilor (Cohn et al 1995), SphygmoCor (Giot and Dcgautc 1996), Pulsepen (Sale et al 2004) and Colin CBM-7000 arterial tonometry (manufactured by the Colin Corporation, Japan) employs the principle of applanation tonometry, where the artery is partially compressed against a hard structure to provide a continuous read-out of the pulse pressure waveform without the use of an occluding cuff. However, in order to calibrate the pressure measurement, the SBP, DBP and MBP in the opposite arm must also be measured by the auscultatory or cuff-oscillometric method for proportionally estimating the intra-arterial blood pressure. Thus two devices must be used at the same time to measure the blood pressures, which is a major drawback for its medical applications. Besides, the transmission characteristics of the blood pressure from the artery to the skin are not linear or constant.
Finapres (Boehmer 1987) or Portpres is a non-invasive continuous finger arterial blood pressure monitor based on the vascular unloading technique. With the volume-clamp method of Pen{tilde over ( )}n´az, the finger arteries are clamped at a fixed diameter by applying an external pulsating pressure via an inflatable bladder mounted in a finger cuff and a fast-acting servo system. Finapres uses the criteria of Wesseling for determination of the setpoint. The diameter at which the finger arteries are clamped is determined from an infrared plethysmograph mounted in the finger cuff, such that the transmural pressure is zero and the cuff pressure is then equal to the intra-arterial pressure by assuming the ideal transmission of the pulsating pressure from the artery to the cuff. Later, Yamakoshi et al proposed the volume-compensation method to improve the servo reference, and developed a local pressurization technique to design a pad-type cuff sphygmomanometer (Tanaka et al 2005) for finger and wrist to avoid the occluding cuff encircling the biological segment that makes it uncomfortable in long-term measurements.
In summary, none of the above methods measures the actual instantaneous blood pressure in an artery, mainly because these methods are not capable of determining the transmission characteristics of the tissue and blood vessel. A possible approach to obtain these characteristics is to decouple the pulsation of the blood vessel from the tissues by using control and identification techniques.
Methods to Detect the Arterial Compliance
The cardiovascular diseases are mainly caused from arterial angiosclerosis. The estimation of the arterial angiosclerosis is primarily related the compliance of blood vessel. However, it is difficult to directly measure the compliance directly at present, because computing the compliance needs two signals of pressure and the variation of vascular volume (or diameter) simultaneously, and it is not easy to place two sensors to acquire signals at same measuring point. Hence, the present non-invasive techniques to detect the degree of arterial angiosclerosis mainly involve measuring arterial blood pressure, ABI (Ankle Branch Index), PWV (Pulse Wave Velocity) and AEI (Artery Elasticity Index).
The most common diagnosis for hypertension disorder is the measurement of systolic blood pressure (SBP) and diastolic blood pressure (DBP) of arterial blood pressure; ABI (Ankle-Brachial Index) is typically used for assessing the vascular obstruction at lower extremities and suitable for detecting the vascular obstruction caused by thrombus (atherosclerosis); in case of PWV (Pulse Wave Velocity), based on the time reference provided by electrocardiogram (ECG), pulse pressure waveforms of arteries at two electrodes are captured respectively, and the arterial wave velocity for assessing the level of atherosclerosis can be measured via time difference between two pulse waves; and for AEI (Artery Elasticity Index), it uses the continuous blood pressure via modified Windkessel model to compute the Large Artery Elasticity Index (Capacitive Arterial Compliance) C1 and the Small Artery Elasticity Index (Reflective Arterial Compliance) C2. It is obvious that the abovementioned measuring techniques are based on continuous blood pressure, not directly detect the arterial compliance. Besides, in order to accurate measuring the compliance of blood vessel, not only to have pressure and variation of vascular volume (or diameter) signals, but also the pulsation of blood vessel should not be affected by surrounding tissue.
In 2007, the present inventor developed an innovative blood pressure measurement technique, named TCM (Tissue Control Method), which by maintaining the DC part of blood pressure and tracking the AC part of blood pressure to cause the vascular truly unloading; the blood vessel is pulsated without the effect of surrounding tissue. Accordingly, the variation of vascular diameter is obtained, but meanwhile, the AC part of blood pressure is lost, that is the reference pressure for controller is absent as well. For estimating vascular impedance, the self-adaptive control algorithm is adopted, and the peak-to-peak blood pressure of the previous pulse is taken as reference pressure, by which, the beat-based intravascular continuous blood pressure could be obtained, but obviously the value is inaccurate, as shown in FIG. 11; in addition, the obtained arterial vascular impedance is just an approximate mean value in one pulse pressure cycle, as shown in FIG. 10. In other words, although TCM method could lead to the unloading state of the blood vessel, the absence of reference pressure causes the actual continuous blood pressure and the dynamic compliance of the blood vessel to be unobtainable. The present inventor then proposes a real-time based decoupling technique, named VLDT (Vascular Unloading Decoupling Technique), to measure the actual continuous pressure of fluid in the elastic tube and the dynamic compliance of elastic tube as well.